Two: The firm charges a management fee based on committed capital. The typical rate is 2% per year. So, if they're running a $100 million fund, they transfer $2 million from the limited partners (the investors) to themselves every year. The management fee theoretically pays for salaries and those costs which can't get passed to startups the firm invests in. (I say "theoretically" because there is an inflection point in the hundreds of millions in fund size range where the management fee is so large that one cannot possibly spend it all except by increasing salaries, such that partners no longer need to be successful in investing to do well.)
Twenty: VCs receive a percentage of the "carry", which is the capital gains on the fund's investment. My understanding is this is typically due upon an individual investment's exit or the end of the fund's life, whichever comes first, but don't trust me on the micromechanics of that. If a VC firm invests $20 million in a company and it later sells for $60 million (their share), the gain is $40 million, so $8 million gets taken for the VC firm and then $32 million is split by the limited partners.
This is compensation for firms. Compensation for partners and associates are handled differently. Typically, associates are on straight salary. (Want a ballpark number? "Well-off lawyer.") Partners typically both have a salary (Want a ballpark number? "More than anyone in the Valley who is not CEO of a public company.") and take home a percentage of the carry (the twenty). Which partners get what percentage is a matter highly specific to the firm's internal structure: suffice it to say that it is good to be king in a lot of places.
Much like law firms, the main form of professional advancement at VC firms is to claw one's way from associate to partner, at which point things get radically more financially rewarding. (The alternative is to build up relationships with wealthy people and then go into business for yourself, by convincing 5 to 20 people to stake you with a few million dollars apiece.)
n.b. It is common for senior partners in the VC fund, particularly those who have successfully lead funds before (many VC firms manage multiple funds, each with a 10 year life and, say, 3-4 years between start dates), to have money invested in the fund themselves. This is generally a fairly modest amount of money relative to fund size.